Arterial baroreflex control of heart rate during exercise in postural tachycardia syndrome

Postural Tachycardia Syndrome: Beyond Orthostatic Intolerance

Postural tachycardia syndrome (POTS) is a form of chronic orthostatic intolerance for which the hallmark physiological trait is an excessive increase in heart rate with assumption of upright posture. The orthostatic tachycardia occurs in the absence of orthostatic hypotension and is associated with a >6-month history of symptoms that are relieved by recumbence. The heart rate abnormality and orthostatic symptoms should not be caused by medications that impair autonomic regulation or by debilitating disorders that can cause tachycardia. POTS is a "final common pathway" for a number of overlapping pathophysiologies, including an autonomic neuropathy in the lower body, hypovolemia, elevated sympathetic tone, mast cell activation, deconditioning, and autoantibodies. Not only may patients be affected by more than one of these pathophysiologies but also the phenotype of POTS has similarities to a number of other disorders, e.g., chronic fatigue syndrome, Ehlers-Danlos syndrome, vasovagal syncope, and inappropriate sinus tachycardia. POTS can be treated with a combination of non-pharmacological approaches, a structured exercise training program, and often some pharmacological support.

Menstrual cycle phase does not affect sympathetic neural activity in women with postural orthostatic tachycardia syndrome

KEY POINTS

Women with the postural orthostatic tachycardia syndrome (POTS) report fluctuations in orthostatic tolerance throughout the menstrual cycle. The mechanism(s) underlying blood pressure control across the menstrual cycle in women with POTS are unknown. The findings of the present study indicate that the menstrual cycle does not affect muscle sympathetic nerve activity but modulates blood pressure and vasoconstriction in POTS women during orthostatic stress. Factors other than sympathetic neural activity are likely responsible for the symptoms of orthostatic intolerance across the menstrual cycle in women with POTS.

ABSTRACT

Patients with the postural orthostatic tachycardia syndrome (POTS) are primarily premenopausal women, which may be attributed to female sex hormones. We tested the hypothesis that hormonal fluctuations of the menstrual cycle alter sympathetic neural activity and orthostatic tolerance in POTS women. Ten POTS women were studied during the early follicular (EF) and mid-luteal (ML) phases of the menstrual cycle. Haemodynamics and muscle sympathetic nerve activity (MSNA) were measured when supine, during 60 deg upright tilt for 45 min or until presyncope, and during the cold pressor test (CPT) and Valsalva manoeuvres. Blood pressure and total peripheral resistance were higher during rest and tilting in the ML than EF phase; however, heart rate, stroke volume and cardiac output were similar between phases. There were no mean ± SD differences in MSNA burst frequency (8 ± 8 EF phase vs. 10 ± 10 bursts min(-1) ML phase at rest; 34 ± 15 EF phase vs. 36 ± 16 bursts min(-1) ML phase at 5 min tilt), burst incidence or total activity, nor any differences in the cardiovagal and sympathetic baroreflex sensitivities between phases under any condition. The incidence of presyncope was also the same between phases. There were no differences in haemodynamic or sympathetic responses to CPT or Valsalva. These results suggest that the menstrual cycle does not affect sympathetic neural activity but modulates blood pressure and vasoconstriction in POTS women during tilting. Thus, factors other than sympathetic neural activity are probably responsible for the symptoms of orthostatic intolerance across the menstrual cycle in women with POTS.

Exercise in the postural orthostatic tachycardia syndrome

Patients with the Postural Orthostatic Tachycardia Syndrome (POTS) have orthostatic intolerance, as well as exercise intolerance. Peak oxygen uptake (VO2peak) is generally lower in these patients compared with healthy sedentary individuals, suggesting a lower physical fitness level. During acute exercise, POTS patients have an excessive increase in heart rate and reduced stroke volume for each level of absolute workload; however, when expressed at relative workload (%VO2peak), there is no difference in the heart rate response between patients and healthy individuals. The relationship between cardiac output and VO2 is similar between POTS patients and healthy individuals. Short-term (i.e., 3 months) exercise training increases cardiac size and mass, blood volume, and VO2peak in POTS patients. Exercise performance is improved after training. Specifically, stroke volume is greater and heart rate is lower at any given VO2 during exercise after training versus before training. Peak heart rate is the same but peak stroke volume and cardiac output are greater after training. Heart rate recovery from peak exercise is significantly faster after training, indicating an improvement in autonomic circulatory control. These results suggest that patients with POTS have no intrinsic abnormality of heart rate regulation during exercise. The tachycardia in POTS is due to a reduced stroke volume. Cardiac remodeling and blood volume expansion associated with exercise training increase physical fitness and improve exercise performance in these patients.

Usefulness of plasma copeptin as a biomarker to predict the therapeutic effectiveness of metoprolol for postural tachycardia syndrome in children

Metoprolol is clinically used to treat postural tachycardia syndrome (POTS) in children, but its effectiveness is unsatisfactory. Biomarkers to predict therapeutic efficacy are needed. We aimed to explore changes in the plasma copeptin level for assessing the therapeutic efficacy of metoprolol for POTS in children. We included 49 children with POTS and 25 healthy children as controls. Patients received metoprolol for 1.5 to 3 months. The plasma copeptin level was measured by sandwich immunoluminometric assay. The area under the receiver operating characteristic curve was used to explore the predictive value of the plasma copeptin level. The baseline plasma copeptin level was higher in children with POTS than controls (10.524 ± 2.016 vs 8.750 ± 1.419 pmol/L, p <0.001) and was lower for responders than nonresponders to metoprolol (9.377 ± 1.411 vs 12.054 ± 1.662 pmol/L, p = 0.003). The area under the receiver operating characteristic curve was 0.889 (95% confidence interval 0.799 to 0.980). With a baseline plasma copeptin level of 10.225 pmol/L as a cutoff, the sensitivity was 90.5% and specificity 78.6% in predicting the efficacy of metoprolol in children with POTS. In conclusion, the baseline plasma copeptin level can be used as a biomarker to predict the therapeutic effectiveness of metoprolol in children with POTS.

Diagnosis and management of postural orthostatic tachycardia syndrome: A brief review

Postural orthostatic tachycardia syndrome (POTS) has been recognized since at least 1940. A review of the literature identifies differences in the definition for this condition and wide variations in treatment and outcomes. This syndrome appears to describe a group of conditions with differing pathophysiology, which requires treatment tailored to the true underlying disorder. Patients need to be fully evaluated to guide treatment. Further research is required to effectively classify the range of underlying pathophysiology that can produce this syndrome and to guide optimal management.

Postural orthostatic tachycardia syndrome and general anesthesia: a series of 13 cases

STUDY OBJECTIVE

To investigate whether patients with postural orthostatic tachycardia syndrome (POTS) developed unexpected perioperative complications.

DESIGN

Retrospective case series.

SETTING

Academic medical center.

MEASUREMENTS

The records of 13 patients with POTS, who underwent surgical procedures during general anesthesia, were studied. Details of disease management, anesthetic induction, hemodynamic response to induction and intubation, intraoperative course, and immediate postoperative management were analyzed.

MAIN RESULTS

Three patients developed prolonged intraoperative hypotension, which was not associated with induction of anesthesia. All 13 patients were successfully treated and they recovered without complications. There were no unplanned hospital or intensive care admissions.

CONCLUSIONS

Intraoperative hypotension, but not tachycardia, was observed in three of 13 patients with POTS who received general anesthesia for a variety of surgical procedures using multiple medications and techniques.

Exercise performance in adolescents with autonomic dysfunction

OBJECTIVE

To test the hypothesis that excessive postural tachycardia is associated with deconditioning rather than merely being an independent sign of autonomic dysfunction in patients with postural orthostatic tachycardia syndrome (POTS).

STUDY DESIGN

We retrospectively analyzed records from 202 adolescents who underwent both head up-tilt and maximal exercise testing. Patients were classified as POTS if they had ≥ 30 min(-1) rise in heart rate (HR) after tilt-table test; and deconditioned if peak O(2) uptake was < 80% predicted. Changes in HR during exercise and recovery were compared between groups.

RESULTS

Two-thirds of patients were deconditioned, irrespective of whether they fulfilled diagnostic criteria for POTS, but peak O(2) uptake among patients with POTS was similar to patients without POTS. HR was higher at rest and during exercise; whereas stroke volume was lower during exercise, and HR recovery was slower in patients with POTS compared with patients without POTS.

CONCLUSIONS

Most patients who presented with chronic symptoms of dizziness, fatigue, or pre-syncope, were deconditioned, but, because the proportion of deconditioned patients was similar in POTS vs non-POTS groups, we conclude that HR changes in POTS are not solely because of inactivity resulting in deconditioning.

Cardiac origins of the postural orthostatic tachycardia syndrome

OBJECTIVES

The purpose of this study was to test the hypothesis that a small heart coupled with reduced blood volume contributes to the postural orthostatic tachycardia syndrome (POTS) and that exercise training improves this syndrome.

BACKGROUND

Patients with POTS have marked increases in heart rate during orthostasis. However, the underlying mechanisms are unknown and the effective therapy is uncertain.

METHODS

Twenty-seven POTS patients underwent autonomic function tests, cardiac magnetic resonance imaging, and blood volume measurements. Twenty-five of them participated in a 3-month specially designed exercise training program with 19 completing the program; these patients were re-evaluated after training. Results were compared with those of 16 healthy controls.

RESULTS

Upright heart rate and total peripheral resistance were greater, whereas stroke volume and cardiac output were smaller in patients than in controls. Baroreflex function was similar between groups. Left ventricular mass (median [25th, 75th percentiles], 1.26 g/kg [1.12, 1.37 g/kg] vs. 1.45 g/kg [1.34, 1.57 g/kg]; p < 0.01) and blood volume (60 ml/kg [54, 64 ml/kg] vs. 71 ml/kg [65, 78 ml/kg]; p < 0.01) were smaller in patients than in controls. Exercise training increased left ventricular mass and blood volume by approximately 12% and approximately 7% and decreased upright heart rate by 9 beats/min [1, 17 beats/min]. Ten of 19 patients no longer met POTS criteria after training, whereas patient quality of life assessed by the 36-item Short-Form Health Survey was improved in all patients after training.

CONCLUSIONS

Autonomic function was intact in POTS patients. The marked tachycardia during orthostasis was attributable to a small heart coupled with reduced blood volume. Exercise training improved or even cured this syndrome in most patients. It seems reasonable to offer POTS a new name based on its underlying pathophysiology, the "Grinch syndrome," because in this famous children's book by Dr. Seuss, the main character had a heart that was "two sizes too small."

Initial orthostatic hypotension is unrelated to orthostatic tolerance in healthy young subjects

The physiological challenge of standing upright is evidenced by temporary symptoms of light-headedness, dizziness, and nausea. It is not known, however, if initial orthostatic hypotension (IOH) and related symptoms associated with standing are related to the occurrence of syncope. Since IOH reflects immediate and temporary adjustments compared with the sustained adjustments during orthostatic stress, we anticipated that the severity of IOH would be unrelated to syncope. Following a standardized period of supine rest, healthy volunteers [ n = 46; 25 ± 5 yr old (mean ± SD)] were instructed to stand upright for 3 min, followed by 60° head-up tilt with lower-body negative pressure in 5-min increments of −10 mmHg, until presyncope. Beat-to-beat blood pressure (radial arterial or Finometer), middle cerebral artery blood velocity (MCAv), end-tidal Pco2, and cerebral oxygenation (near-infrared spectroscopy) were recorded continuously. At presyncope, although the reductions in mean arterial pressure, MCAv, and cerebral oxygenation were similar to those during IOH (40 ± 11 vs. 43 ± 12%; 36 ± 18 vs. 35 ± 13%; and 6 ± 5 vs. 4 ± 2%, respectively), the reduction in end-tidal CO2 was greater (−7 ± 6 vs. −4 ± 3 mmHg) and was related to the decline in MCAv ( R2 = 0.4; P < 0.05). While MCAv pulsatility was elevated with IOH, it was reduced at presyncope ( P < 0.05). The cardiorespiratory and cerebrovascular changes during IOH were unrelated to those at presyncope, and interestingly, there was no relationship between the hemodynamic changes and the incidence of subjective symptoms in either scenario. During IOH, the transient nature of physiological changes can be well tolerated; however, potentially mediated by a reduced MCAv pulsatility and greater degree of hypocapnic-induced cerebral vasoconstriction, when comparable changes are sustained, the development of syncope is imminent.

Cardiovascular dynamics in healthy subjects with differing heart rate responses to tilt

Orthostatic stress such as head-up tilt (HUT) elicits a wide range of heart rate (HR) and arterial pressure (AP) responses among healthy individuals. In this study, we evaluated cardiovascular dynamics in healthy subjects with different HR responses to HUT, but without autonomic dysfunction. We measured AP (brachial artery) and HR (ECG) during 5 min of 60 degrees HUT in 76 healthy normotensive individuals. We then chose individuals on the basis of the extremes of HR responses to HUT (high = DeltaHR > or = 20 beats/min, and low = DeltaHR < or = 10 beats/min; n = 15 per group). Peak HR during HUT was 87 +/- 10 beats/min in the high and 69 +/- 14 beats/min in the low group (P < 0.05). High HR responders had lower systolic pressure at baseline (121 +/- 9 vs. 129 +/- 11 mmHg, P < 0.05) and during HUT (120 +/- 10 vs. 131 +/- 13 mmHg, P < 0.05), and higher plasma norepinephrine (NE) response to HUT (DeltaNE: 156.9 +/- 17.8 vs. 89.0 +/- 17.2 pg/ml; P < 0.05). DeltaNE during HUT was also significantly correlated with DeltaHR when all 76 subjects were included in a regression analysis (r = 0.39; P < 0.001). Pulse pressure was lower during HUT in high HR responders compared with low HR responders (45 +/- 1 vs. 55 +/- 2 mmHg, P < 0.05). High HR responders also had larger fluctuations in systolic and pulse pressure during HUT (coefficient of variation = 10.7 +/- 0.7 vs. 5.7 +/- 0.3%; 7.9 +/- 0.5 vs. 4.1 +/- 0.4%, respectively, P < 0.05). Sex distribution was different between groups (high: 5 women, 10 men; low: 10 women, 5 men). Higher HR with lower AP during HUT is consistent with normal baroreflex mechanisms of integration. Although interindividual variability appears to be a fundamental part of cardiovascular regulation, the mechanisms of these differences and the sex discrepancy requires further investigation.

POTS versus deconditioning: the same or different?

The 2007 Streeten Lecture focused on the idea that physical deconditioning plays a key role in the symptomology and pathophysiology of POTS. Parallels were drawn between the physiological responses to orthostatic stress seen in POTS patients and the physiological responses seen in "normal" humans after prolonged periods of bedrest, deconditioning, or space flight. Additionally, the idea that endurance exercise training might ameliorate some of these symptoms was also advanced. Finally, potential parallels between POTS, chronic fatigue syndrome, and fibromyalgia were also drawn and the potential role of exercise training as a "therapeutic intervention" in all three conditions was raised. The conceptual model for the lecture was that after some "initiating event" chronic deconditioning plays a significant role in the pathophysiology of these conditions, and these physiological changes in conjunction with "somatic hypervigilence" explain many of the complaints that this diverse group of patients have. Additionally, the idea that systematic endurance exercise training might be helpful was advanced, and data supportive of this idea was reviewed. The main conclusion is that the medical community must retain their empathy for patients with unusual conditions but at the same time send a firm but empowering message about physical activity. As always, we must also ask what do the ideas about physical activity and inactivity and the conditions mentioned above not explain?

Reduced stroke volume during exercise in postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is characterized by excessive tachycardia without hypotension during orthostasis. Most POTS patients also report exercise intolerance. To assess cardiovascular regulation during exercise in POTS, patients (n = 13) and healthy controls (n = 10) performed graded cycle exercise at 25, 50, and 75 W in both supine and upright positions while arterial pressure (arterial catheter), heart rate (HR; measured by ECG), and cardiac output (open-circuit acetylene breathing) were measured. In both positions, mean arterial pressure, cardiac output, and total peripheral resistance at rest and during exercise were similar in patients and controls (P > 0.05). However, supine stroke volume (SV) tended to be lower in the patients than controls at rest (99 +/- 5 vs. 110 +/- 9 ml) and during 75-W exercise (97 +/- 5 vs. 111 +/- 7 ml) (P = 0.07), and HR was higher in the patients than controls at rest (76 +/- 3 vs. 62 +/- 4 beats/min) and during 75-W exercise (127 +/- 3 vs. 114 +/- 5 beats/min) (both P < 0.01). Upright SV was significantly lower in the patients than controls at rest (57 +/- 3 vs. 81 +/- 6 ml) and during 75-W exercise (70 +/- 4 vs. 94 +/- 6 ml) (both P < 0.01), and HR was much higher in the patients than controls at rest (103 +/- 3 vs. 81 +/- 4 beats/min) and during 75-W exercise (164 +/- 3 vs. 131 +/- 7 beats/min) (both P < 0.001). The change (upright - supine) in SV was inversely correlated with the change in HR for all participants at rest (R(2) = 0.32), at 25 W (R(2) = 0.49), 50 W (R(2) = 0.60), and 75 W (R(2) = 0.32) (P < 0.01). These results suggest that greater elevation in HR in POTS patients during exercise, especially while upright, was secondary to reduced SV and associated with exercise intolerance.